A. Field of the Invention
The present invention relates to an upper surface blowing powered lift system in a STOL type aircraft.
B. Brief Description of the Prior Art
In recent years, increasing attention has been given to aircraft designs having the capability of efficient cruise at relatively high speeds, and also having the ability to take off and land in relatively short distances. Such aircraft are generally referred to as "STOL aircraft" (i.e. short take-off and landing aircraft). To develop adequate lift for the aircraft at low speed operation, such STOL aircraft commonly utilize the jet exhaust from the engines in combination with a flap system on the wing to divert the exhaust downwardly and thus increase lift.
One method of doing this is by blowing the jet exhaust over the upper surface of the wing, and utilizing downwardly extending trailing edge flaps to divert the exhaust downwardly by the Coanda effect. One such aircraft is shown in U.S. Pat. No. 3,977,630, Lewis et al, this patent being assigned to the assignee of the present invention. In the apparatus shown in that patent there is a high mounted wing, and a pair of mixed flow turbofan engines mounted on the wing to discharge their exhaust streams corewise over the upper surface of the wing. There is a pair of trailing edge flaps, one behind each engine. The more forward flap of each pair is mounted on a first arm for rotation in a circular arc about a first axis of rotation. The rear flap of each pair is mounted to a second arm which is in turn mounted for rotation in a circular arc about an axis on the first arm which supports the forward flap.
Each pair of flaps has a stowed position where the flaps fit into a cove section at the aft end of the wing, and a deployed position where the two flaps are rotated downwardly and rearwardly. At the forward end of each flap is a small panel, with these panels being able to be moved to an upper position where they form with the upper surface of the wing and the upper surface of the two flaps a continuous aerodynamic surface. These two panels can also be moved to a down position to open slots at the forward ends of their two flaps. Thus, in the event of a loss of power, the panels of the flaps can be moved to the position to open the slots, thus decreasing drag on the wing.
With the two pairs of flaps in their fully deployed position, and with the flap panels closed, they define a downwardly and rearwardly curving aerodynamic surface at the trailing edge of the wing. The jet exhaust being discharged over the upper rear edge of the surface of the right and left wing sections follows this downwardly curved aerodynamic contour provided by the two pairs of flaps due to the Coanda effect, to develop a downward thrust.
Several years ago, experimental work was done relative to upper surface blown flaps, such as those described above, with this work being reported in a publication of the National Aeronautics and Space Administration, NASA SP-320, entitled "STOL Technology", further entitled "A Conference Held at Ames Research Center, Moffett Field, Calif., Oct. 17-19, 1971". In that publication, there are disclosed upper surface blown flap configurations where the radius of curvature at the forward portion of the flap is relatively small, and the radius of curvature is greater in the rear portion of the flap. However, it was found that with that particular flap configuration, it was necessary to "flatten" the jet exhaust so that the exhaust stream passing over the upper surfaces of the trailing edge flaps was relatively thin. This was necessary to enable the jet exhaust to follow the upper surface of the flap. While this "flattened" configuration of the jet was adequate for STOL operation, it produced too much drag for cruise operation, and therefore was less than totally satisfactory.
Another approach to an upper surface blown flap configuration is disclosed in a paper entitled "Propulsion Integration for a Hybrid Propulsive-Lift System," authored by M. K. Bowden, J. H. Renshaw, and H. S. Sweet. This paper was published by the Society of Automotive Engineers, and was presented at the Air Transportation Meeting, Dallas, Tex., Apr. 30-May 2, 1974. The paper bears a numerical designation "740471."
This paper describes a "hybrid power lift" system in which a portion of the fan air is directed through the interior of the wing to be blown over the upper surfaces of the trailing edge flaps of the wing. In addition, the exhaust from the engine is blown over the upper wing surface. Two flap configurations are shown, but the apparatus actually tested was significantly different. In one configuration, the upper surface of the flap has a more constant radius of curvature from the forward to the rear end. In a second configuration, the flap is identified as an "expanding Jacobs-Hurkamp flap" which has a sharper curvature at the knee point of the flap (i.e. a smaller radius of curvature), and a curvature having a substantially greater radius at the aft portion of the flap. An analysis of the data available concerning this arrangement indicates that the two flap configurations were competitive with one another and there was no particular advantage of using one as opposed to the other.
Possibly the prior art most relevant to the present invention is the YC-14 airplane developed by the Boeing Company, the assignee of the present invention. The main features of this airplane are described in "AIAA Paper No. 75-1015", entitled "Aerodynamic Design of the Boeing YC-14 Advanced Medium STOL Transport (AMST)", authored by Fred W. May and George E. Bean. This paper is believed to have been presented at the AIAA 1975 Aircraft Systems and Technology Meeting at Los Angeles, Calif., Aug. 4-7, 1975. Other aspects of the YC-14 airplane are described in a publication of the Society of Automotive Engineers, entitled "Nozzle Development for the Upper Surface Blown Jet Flap on the YC-14 airplane", authored by Howard Skavdahl, Timothy Wang, and William J. Hirt. It is believed that this paper was presented at the air transportation meeting, Dallas, Tex., Apr. 30-May 2, 1974.
The YC-14 uses the upper surface blown concept generally as described in the aforementioned U.S. Pat. No. 3,977,630, Lawis et al. This airplane has a twin engine, high wing configuration. The two engines are mounted on the wing very close to the fuselage (known as a "shoulder mount") with the jet exhaust passing over the upper surface of the wing. There is a constant section, two segment, upper surface blown flap behind each engine. Also, there are tapered, double slotted mechanical flaps outboard of the upper surface blown flaps.
The two engines of the YC-14 have a mixed flow exhaust nozzle (i.e. where the primary and fan flow are combined), with the nozzle having a door on the outboard side to allow jet exhaust to spread outboard, thereby thinning the jet and increasing its span for better turning by the Coanda effect. The door is closed in cruise to maintain a narrower jet width, thus reducing interference and scrubbing drag.
Behind each jet nozzle, there are four retractable vortex generators which are used to aid in turning the jet exhaust downwardly over the upper surface blown flap at higher deflection angles. In the AIAA paper noted above, it is noted that the Coanda turning is enhanced by the presence of the vortex generators.
The two sections of each upper surface blown flap rotate about a simple hinge and form a sealed continuous surface behind the exhaust nozzle when these sections are in their downwardly and rearwardly extending position. In any of the deployed positions, the upper surface of the blown flap is designed to approximate a circular arc of 68" radius, which is about twice the maximum height of the nozzle exit. In the event of engine failure with the upper surface blown flaps in their extended position, the forward flap segment behind the failed engine will rock on a pivot to transform the flap into a conventional double slotted flap.
One of the major considerations in designing an upper surface blown flap system is the ability to turn the jet exhaust at full deflection of the upper surface blown flaps. Early development of upper surface blown flap technology concentrated on methods of reducing jet thickness and increasing radius of flap curvature. As mentioned in early literature, Coanda flow turning depends strongly on the ratio of jet thickness to turning radius. For a given jet Mach number, a maximum ratio exists above which flow turning is poor. This early observation can be explained by elementary fluid mechanics as follows. Poor turning is a result of flow separation when the boundary layer can no longer overcome the retarding force of an adverse pressure gradient along the flap surface. The magnitude of the adverse pressure gradient increases with increasing jet thickness. Thus, one way to avoid flow separation (i.e. achieve good turning) is to thin down the jet and reduce the adverse pressure gradient.
However, while a thin jet is desirable for proper turning with full flap deflection, it is not desirable for cruise mode because of the high drag resulting from the thin jet passing over the air foil. While the prior art developments noted above (i.e. the hybrid propulsive lift system and the Boeing YC-14 upper surface blown life system) do represent advances in the state of the art in that improved jet turning was achieved, prior to the present invention, to the best knowledge of the applicant, proper turning of the jet under conditions of full flap deflection could not be achieved without thinning the jet to an extent that is less than totally desirable for cruise mode. As will be disclosed later herein, the upper surface blown flap system of the present invention permits the use of a thicker jet exhaust (which in turn means reduced drag) to permit greater efficiencies in the cruise mode.
With regard to the overall operating characteristics of an upper surface blown flap system, there are in general three major considerations. First, there is the important consideration of the flap system being able to develop adequate lift to accomplish the design objectives of the aircraft, and this is directly related to the ability to turn the jet exhaust over the upper surface blown flap. Second, since the augmented lift developed by the flaps in their deployed position results from the downward deflection of the jet exhaust at the trailing edge of the wing, there is the consideration that the center of lift in the wing will to some extent move rearwardly in the STOL mode of operation and thus develop a pitching moment. This pitching moment is usually counteracted by using the horizontal tail and the elevators of the aircraft to develop a counter moment. To optimize the control characteristics of the airplane it is more desirable that any shift in the center of lift in the wing be kept to a practical minimum.
A third consideration is the capability of utilizing the flaps in a manner to control the aircraft on its glide path. Since the jet engine does not always respond with sufficient rapidity to move from a lower to a higher power setting or vice versa, the flaps of an airplane are utilized to increase or decrease lift, or vary the drag on the wing to maintain the plane on the proper glide path. The upper surface blown flaps in a STOL aircraft can be used for this purpose, since they can effectively vary the rearward and downward thrust components by moving the flaps to different positions. Thus, an important consideration becomes the ability of the upper surface blown flaps in a STOL aircraft to respond in a manner to provide desired control characteristics for the aircraft.
While the apparatus described above does represent advances in the art, there is continuing effort to make improvements. Thus the present invention has for its main objective, the improvement in the performance of a flap system utilized for an aircraft having an upper surface blown powered lift system such as those described above.
With regard to the prior art disclosed from a search of the patent literature, the following are noted:
U.S. Pat. No. 2,334,070 discloses broadly the concept of blowing air over an upwardly facing convexly curved aerodynamic surface to improve lift.
U.S. Pat. No. 2,386,987, Stalker, discloses a flap system in which there are slots at the leading edge of the flaps. Air can either be blown in or sucked out of the slots. U.S. Pat. No. 2,449,022, Stalker, shows a flap system where air is sucked into the flaps so as to reduce drag.
U.S. Pat. No. 2,517,524, Beck et al, discloses a flap system where boundary layer air is sucked away for an improved aerodynamic effect.
U.S. Pat. No. 2,555,862, Romani, discloses a particular configuration of a trailing edge flap for wings having high sweep angles. This patent does not deal with the problem of augmented lift by upper surface blowing.
U.S. Pat. No. 2,844,337, MacArthur et al, shows a system where air is blown over the upper surface of a trailing edge flap to remove an inert boundary layer from the flap.
U.S. Pat. No. 2,876,966, Cook, shows a flap system where suction is applied to the surface of the flap through a porous surface.
U.S. Pat. No. 2,910,254, Razak, shows a byplane where air is sucked over the lower wing flap and blown over the upper wing flap.
U.S. Pat. No. 2,978,207, Davidson, discloses the concept of mounting an engine within the wing, and discharging the exhaust directly over the trailing edge flap, without first passing the exhaust over the upper surface of the main wing portion. The flap has a uniformly curved forward surface portion and a substantially straight surface contour rearwardly of the forward curved portion.
U.S. Pat. No. 3,012,740, Wagner, relates to an aircraft boundary layer control system. Air is drawn in at one portion of the wing and discharged over the wing at another portion.
U.S. Pat. No. 3,018,983, Davidson, shows an upper surface blown flap system where diverters can be placed rearwardly of the jet exhaust to deflect it downwardly.
U.S. Pat. No. 3,139,248, Alvarez-Calderon, discloses a wing which can have its spanwise length varied by pivotally mounting the outside portion of the wing about a cordwise axis. Thus, the two outside wing portions can be swung downwardly and inwardly to a position beneath the two inboard wing sections to form two wing members of lesser spanwise length. The two outboard wing sections can be swung outwardly to their outwardly deployed position to form wings of greater spanwise length.
U.S. Pat. No. 3,259,341, Steidl, discloses a concept for an airfoil where air is blown from within the wing over the upper surface of a trailing edge flap.
U.S. Pat. No. 3,347,495, Eberhartd et al, discloses an aircraft where flow is directed over the upper surface of a trailing edge flap. Compressed air is directed into ejector nozzles which in turn direct the air over the trailing edge flap.
U.S. Pat. No. 3,438,599, Welzen, discloses an airfoil with a trailing edge flap which is movable on a curved track from a stowed to a deployed position. The patent does not relate to upper surface blowing over a trailing edge flap.
U.S. Pat. No. 3,614,028, Kleckner, discloses a STOL aircraft where a portion of the efflux from the fan section of a jet engine passes over the wing of the aircraft, while the main discharge of the jet engine passes under the wing. A trailing edge flap is utilized to direct the jet flow downwardly for STOL operation.
U.S. Pat. No. 3,756,540, Williams, discloses an airfoil which is divided into five sections along the cord length, each section producing a specialized flow with respect to the aerodynamic surface. The trailing edge is formed in a Coanda profile, and a tangential jet slot is provided to go over and around the Coanda profile to prevent flow separation and move the stagnation region aft on the wing.
U.S. Pat. No. 3,778,009, Jones, discloses a trailing edge device having a pair of members which can be extended or inflated to produce what is described generally as a semi-cylindrical profile. Air is blown over this semi-cylindrical profile from one or more naturally or forcibly blown slots to achieve boundary layer control.
U.S. Pat. Nos. 3,837,601, and 3,987,983, the inventor in both of these patents being James B. Cole, both disclose trailing edge flap configurations generally similar to those disclosed in the above-mentioned U.S. Pat. No. 3,977,630, Lewis et al patent.
U.S. Pat. No. 3,971,534, Grotz, discloses an upper surface blown flap system where two sets of vanes are positioned immediately rearwardly of the jet nozzle to deflect the jet flow outwardly.
U.S. Pat. No. 3,974,987, Shorr, discloses a flap system where compressed air is blown over the upper surface of the flap to help smooth the boundary layer.
U.S. Pat. No. 3,985,319, Dean et al, discloses a linkage system for a pair of trailing edge flaps. Two sets of four bar linkages are provided, one for each flap, and these swing the flaps downwardly and rearwardly to a deployed position.
U.S. Pat. No. 4,019,696, Hart et al, discloses an upper surface blown flap system using vortex generators in a manner somewhat similar to the vortex generators of the YC-14 airplane.
British Patent Specification No. 1,178,312, discloses a flexible trailing edge flap which can be positioned in the manner to achieve the Coanda effect. This patent specification states that the flap should have a substantially uniform curvature for best performance.